Production method of thin wall cordierite honeycomb structure

ABSTRACT

A production method of a honeycomb structure is provided. The production method of a thin wall cordierite honeycomb structure having cordierite as the main component of the crystalline phase includes: adding a forming auxiliary agent to a cordierite material to obtain a mixture, kneading the mixture to obtain a material batch, forming the material batch by extrusion to form a honycomb compact, drying the honeycomb compact to obtain a dried body, and firing the dried body. The cordierite material batch contains 65% by weight or more flat plate-like cordierite raw material including crystal water made from talc, kaolin and aluminum hydroxide with the BET specific surface areas for the cordierite raw materials being 7 to 18 m 2  /g of talc, 14 to 22 m 2  /g of kaolin and 6 to 18 m 2  /g of aluminum hydroxide. The method can realize a thinner wall and a higher cell density and can improve the formability (in particular, the lubricity and the shape stability) at the time of extruding, the mechanical strength (hydrostatic pressure fracture strength), and the catalyst carrying characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing an excellentlystrong thin wall cordierite ceramic honeycomb structure by extruding.

2. Description of Related Art

Cordierite ceramic honeycomb structures are used as an exhaust gaspurifying catalyst carrier, a filter, or a heat exchanger forautomobiles or the industrial use.

Recently, improvement of the thermal shock resistance and the strengthhave been desired particularly for a honeycomb catalyst carrier to beused in an exhaust gas apparatus for automobiles since it needs to bemounted in the vicinity of the engine in order to improve the catalystperformance by reducing the volume of the catalyst carrier, to improvethe light off performance, to lower the pressure loss for improving thefuel consumption performance and improving the output, to improve thestrength for reducing the cost for canning into the casing, and toimprove the catalyst activity.

Therefore, a thinner wall for a rib of a honeycomb structure forimproving the catalyst performance and a thinner wall for a rib of ahoneycomb structure while maintaining the cell density for achieving ahigher cell density and a lower pressure loss have been conventionallydiscussed. However, since materials (in particular, magnesia material)needs to have fine particles due to the strength decline for the thinnerwall in the porous cordierite honeycomb structure and reduction of thedie slit width at the time of extruding, a problem is involved in that adrastic rise in the thermal expansion ratio can be generated.

Furthermore, it is difficult to have cordierite ceramics denser. Inparticular, since the amount of impurities to be a fusing agent such ascalcia, alkaline, and sodium carbonate needs to be extremely small witha low expansion cordierite material showing a 2.0×10⁻⁶ /°C. or less at aroom temperature to 800° C. coefficient of thermal expansion, the glassphase becomes extremely small so as to form a porous substance.

In particular, since a cordierite honeycomb structure recently used asan automobile exhaust gas purifying catalyst carrier requires a 1.5×10⁻⁶/°C. or less coefficient of thermal expansion from a room temperature to800° C., the porosity can be 20 to 45% at best even if refined talc,kaolin, alumina materials with little impurities. In particular, in ahoneycomb structure having a 30% or less porosity, increase of theimpurity amount and finer material particles are necessary so that onehaving a 1.0×10⁻⁶ /°C. or less coefficient of thermal expansion from aroom temperature to 800° C. have not been obtained.

Furthermore, since a cordierite honeycomb structure having acomparatively low porosity contracts drastically in the drying andfiring processes, a crack can be easily generated and thus it isdifficult to obtain a large honeycomb structure with a good yield.

In order to solve the problems, Japanese Patent Publication (Kokoku) No.4-70053 discloses a method of improving the strength of ceramic itselfby having the porosity of cordierite ceramics denser to 30% or less.

In the method, the ceramic itself becomes denser and stronger by havinga 30% or less porosity of the cordierite ceramics in order to preventdecline of the isostatic strength, which is the compression load fromthe outer wall and outer periphery direction of the honeycomb structureby the honeycomb cell deformation generated at the time of extruding.

However, in the above-mentioned method, since the porosity of thecordierite ceramics is 30% or les, the improvement of the catalystcarrying characteristics of the cordierite honeycomb structure cannot beexpected, and further, the effect of having a thinner wall and a highercell density in the cordierite honeycomb structure was not sufficient.

SUMMARY OF THE INVENTION

Accordingly, in order to cope with the above-mentioned conventionalproblems, an object of the present invention is to provide a productionmethod of a cordierite honeycomb structure, capable of achieving athinner wall and a higher cell density in the cordierite honeycombstructure and improving the formability (in particular, the lubricityand the shape stability) at the time of extruding, the mechanicalstrength (hydrostatic pressure fracture strength), and the catalystcarrying characteristics.

According to the present invention, there is provided a productionmethod of a thin wall cordierite honeycomb structure having cordieriteas the main component of the crystalline phase, which comprises: addinga forming auxiliary agent to a cordierite material to obtain a mixture,kneading the mixture to obtain a material batch, forming the materialbatch by extrusion to form a honycomb compact, drying the honeycombcompact to obtain a dried body, and firing the dried body, wherein 65%by weight or more flat plate-like cordierite raw material includingcrystal water made from talc, kaolin and aluminum hydroxide is containedin the cordierite material batch, with the BET specific surface areasfor the cordierite raw materials being 7 to 18 m² /g of talc, 14 to 22m² /g of kaolin and 6 to 18 m² /g of aluminum hydroxide.

In the present invention, it is preferable that the average wallthickness of the obtained honeycomb structure is 110 μm or less, thecoefficient of thermal expansion in the direction of flow passages ofthe cordierite honeycomb structure obtained in the present invention at40 to 800° C. is 0.8×10⁻⁶ /°C. or less, the porosity is 24 to 38%, andthe isostatic strength is 10 kg/cm² or more.

It is further preferable that the BET specific surface areas for thecordierite raw materials being 10 to 18 m² /g of talc, 16 to 20 m² /g ofkaolin and 10 to 18 m² /g of aluminum hydroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM photograph showing the particle structure of talc A.

FIG. 2 is an SEM photograph shwoing the particle structure of talc B.

FIG. 3 is an SEM photograph showing the particle structure of kaolin A.

FIG. 4 is an SEM photograph showing the particle structure of kaolin B.

FIG. 5 is an SEM photograph showing the particle structure of aluminumhydroxide A.

FIG. 6 is an SEM photograph showing the particle structure of aluminumhydroxide B.

FIG. 7 is a photograph showing the texture of the ceramic material,which is the end face structure of the honeycomb structure of Example12.

FIG. 8 is a photograph showing the texture of the ceramic material,which is the end face structure of the honeycomb structure ofComparative Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a production method of a cordierite honeycomb structure according tothe present invention, 65% by weight or more flat plate-like cordieriteraw materials including crystal water made from talc, kaolin andaluminum hydroxide is contained in a cordierite material batch.

Accordingly, the lubricity of the clay mixture to be extruded and theshape stability after extruding can be improved even with a lowextrusion pressure at the time of extruding.

As mentioned above, in the production method of a cordierite honeycombstructure of the present invention, it is important to use the threekinds of raw materials, that is, talc, kaolin and aluminum oxide as thecordierite raw materials at the same time, and to have the cordierie rawmaterials with a flat and plate-like shape in order to reinforce theinteraction in the particle surface, in particular, the surface energyof the flat surface.

Further, the amount of the cordierite raw materials in the cordieritematerial batch is preferably 65% by weight or more, more preferably 70%by weight or more in order to prevent the extrusion pressure rise at thetime of extruding and to improve the shape stability after extruding.

Moreover, since a large amount of calcined talc, calcined kaolin, andalumina leads to drastic deterioration of the clay mixture lubricity, itis preferable to have the minimum amount.

However, although the cordierite raw materials are preferable forextruding of a thin wall honeycomb structure without cell deformation,they can generate a firing crack of a cordierite ceramic honeycombstructure.

Therefore, in a production method of a cordierite honeycomb structure ofthe present invention, the BET specific surface areas of the cordieriteraw materials are defined, that is, 7 to 18 m² /g, preferably 10 to 18m² /g of talc, 14 to 22 m² /g, preferably 16 to 22 m² /g of kaolin and 6to 18 m² /g, preferably 10 to 18 m² /g of aluminum hydroxide.

With the BET specific surface areas of the cordierite raw materialshaving less than 7 m² /g of talc, less than 14 m² /g of kaolin and lessthan 6 m² /g of aluminum hydroxide, the lubricity at the time ofextruding is poor and the interaction among the material particles issmall so that the shape stability after extruding is poor to generatecell deformation, and thus a sufficient isostatic strength (10 kg/cm² ormore) cannot be obtained.

In general, since the aspect ratio of material particles with a smallBET specific surface area is small even when they are fine particles andit is poor in terms of the flatness, the lubricity is poor and thehoneycomb network forming ability is also poor.

On the other hand, with the BET specific surface areas of the cordieriteraw materials having more than 18 m² /g of talc, more than 22 m² /g ofkaolin and more than 18 m² /g of aluminum hydroxide, since the firingcontraction becomes large, it is difficult to prevent crack generationin the firing process.

Further, it is preferable that the talc used as a cordierite rawmaterial of the present invention is a micro talc with a small crystal(single crystal).

The kaolin is preferably delamination kaolin, which has a laminate layerpeeled off to be a thinner layer in the artificial or natural condition,but can be tube-like kaolin to become plate-like at the time ofextruding, such as halloysite.

It is more preferable that the aluminum hydroxide has a high BETspecific surface area applied with a pulverizing treatment to be a thinlayer after crystal precipitation by the Bayer process.

The BET specific surface area herein denotes the surface area per a unitmass of a solid obtained by the gas adsorption theory of Brunauer, Emettand Teller (BET isotherm), which is a method for determining the surfacearea by calculating the area of multimolecular layer.

It is further preferable that the coefficient of thermal expansion at 40to 800° C. in a cordierite honeycomb structure obtained in the presentinvention is 0.8×10⁻⁶ /°C. or less in the direction of flow passages.

With a more than 0.8×10⁻⁶ /°C. coefficient of thermal expansion CTE!(the direction of flow passages of the honeycomb sintered body), thethermal shock resistance Esp! becomes lower than 700° C. for acylindrical honeycomb structure having about 100 mm diameter so that itcannot be used for an automobile exhaust gas catalyst carrier.

The production method of a cordierite honeycomb structure of the presentinvention will be explained in further detail.

The cordierite material batch of a cordierite honeycomb structure isprepared with 65% by weight or more of cordierite raw materialsincluding talc, kaolin, and aluminum hydroxide and the remainderincluding calcined kaolin, alumina, silica, calcined talc, and the likesuch that the chemical composition of the main components can be 42 to56% by weight of SiO₂, 30 to 45% by weight of Al₂ O₃, and 12 to 16% byweight of MgO. A honeycomb compact is formed by adding an organic bindersuch as water and methyl cellulose and a plasticizer to the cordieritematerial batch, mixing, kneading and extruding. A cordierite honeycombstructure can be obtained by drying the honeycomb compact and firing at1350 to 1440° C.

At the time, it is preferable that the temperature rise rate in thecrystal water dehydrating temperature region of the aluminum hydroxideand the kaolin is restrained by 50° C./Hr in order to prevent crackgeneration in the cordierite honeycomb structure. It is also effectiveto conduct a debinder treatment before firing.

As heretofore mentioned, a production method of a cordierite honeycombstructure of the present inveniton has been achieved by finding acordierite raw material having a high BET specific surface area havingboth excellent lubricity at the time of extruding and shape stabilitywith respect to deformation by self-weight after extruding. In producinga thin wall honeycomb structure having a 40 to 110 μm wall thickness,the cell deformation after forming and the coefficient of thermalexpansion after firing can be extremely small, the isostatic strengthwith a 24 to 38% porosity can be 10 kg/cm² or more, and the catalystcarrying characteristics can be improved when the porosity is 30% ormore.

The present invention will be explained in further detail with referenceto examples, but the present invention is not limited thereto.

The performance of the cordierite materials, the honeycomb compacts andthe honeycomb sintered bodies obtained in the examples were evaluated bythe method mentioned below.

(Measuring method for the particle size)

The particle size was measured with Sedigraph (X-ray sedimentationmethod) produced by Micromeritech Corp.

(Measuring method for the BET specific surface area)

The BET specific surface area was measured with Flowsorb II2300 producedby Micromeritech Corp (He 30%!/N₂ 70%! gas was used as the adsorptiongas).

(Mearusing method for the chemical analysis value)

The chemical analysis value was measured by the fluorescent X-rayspectroscopy.

(Measuring method for the porosity)

The porosity was calculated from the entire pore volume in the mercurypenetration method (the cordierite true specific gravity herein was2.52).

(Measuring method for the themal shock resistance).

Whether or not a honeycomb structure (honeycomb sintered body) of a roomtemperature placed in an electric furnace, maintained for 30 minutes,and taken out to the room temperature has a fracture was meacured by thehammering judgment (with 50° C. interval from 600° C., the safetemperature °C! is shown).

(Measuring method for the isostatic strength)

With a honeycomb structure (honeycomb sintered body) inserted in aflexible tube, the pressure (kg/cm²) at which partial breakage wasgenerated by applying a uniform hydraulic pressure was measured (averagevalue of 10 specimens). The isostatic strength can be the criterion forevaluating the endurance, such as the property of holding the honeycombcatalyst to a metal casing and vibration in the actual run.

(Examples 1 to 17, Comparative Examples 1 to 7)

As the cordierite material, clay mixture for extruding was prepared withmaterials having the particle size, the BET specific surface area, andthe chemical analysis value shown in Table 1 according to theproportional ratio shown in Tables 2 and 3 by adding 4% by weight ofmethyl cellulose and water with respect to 100% by weight of thematerial and kneading. The material herein was used after passingthrough a 44 μm sieve.

FIG. 1 is an SEM photograph showing the particle structure of talc A.FIG. 2 is an SEM photograph showing the particle structure of talc B.FIG. 3 is an SEM photograph showing the particle structure of kaolin A.FIG. 4 is an SEM photograph showing the particle structure of kaolin B.FIG. 5 is an SEM photograph showing the particle structure of aluminumhydroxide A. FIG. 6 is an SEM photograph showing the particle structureof aluminum hydroxide B.

                                      TABLE 1    __________________________________________________________________________                   BET               Average                   specific               particle                   surface               size                   area                       Chemical analysis value (%)    Material   (μm)                   (m.sup.2 /g)                       Ig.loss                           SiO.sub.2                              Al.sub.2 O.sub.3                                  MgO                                     TiO.sub.2                                        Fe.sub.2 O.sub.3                                            CaO + Na.sub.2 O + K.sub.2    __________________________________________________________________________                                            O    Talc A     2.9  6.8                        5.7                           61.7                               0.4                                  30.9                                     -- 1.2 0.2    Talc B     3.1 12.5                        5.4                           61.2                               0.6                                  30.9                                     -- 0.9 0.2    Talc C     6.5  7.0                        5.2                           61.7                               0.4                                  30.7                                     -- 1.3 0.2    Talc D     1.3 17.8                        5.3                           61.6                               0.7                                  31.6                                     -- 0.7 0.1    Talc E     3.3 10.0                        5.7                           61.0                               0.7                                  31.1                                     -- 1.0 0.2    Kaolin A   0.4 12.8                       13.9                           45.6                              38.7                                  -- 0.8                                        0.3 0.2    Kaolin B   0.4 19.6                       13.6                           46.3                              38.0                                   0.2                                     0.8                                        0.6 0.2    Kaolin C   0.5 14.0                       14.0                           45.5                              39.0                                  -- 1.0                                        0.4 0.1    Kaolin D   0.4 22.0                       13.5                           45.5                              38.5                                  -- 1.3                                        0.5 0.1    Kaolin E   0.5 16.2                       13.8                           45.7                              38.9                                  -- 0.7                                        0.3 0.1    Calcined talc               3.3  8.5                        0.1                           64.8                               0.7                                  33.0                                     -- 1.2 0.2    Calcined kaolin               1.4  9.3                        0.1                           52.5                              44.6                                  -- 1.5                                        0.3 0.2    Alumina    4.5  0.9                        0.1                           -- 99.5                                  -- -- --  0.3    Silica     3.8  3.5                       --  99.5                               0.4                                  -- -- --  --    Aluminum hydroxide A               1.0  4.8                       33.7                           -- 64.9                                  -- -- --  0.3    Aluminum hydroxide B               0.9 15.8                       34.3                           -- 65.0                                  -- -- --  0.3    Aluminum hydroxide C               0.6 10.3                       33.8                           -- 65.6                                  -- -- --  0.4    Aluminum hydroxide D               0.6 18.0                       34.0                           -- 65.6                                  -- -- --  0.3    Aluminum hydroxide E               1.0  6.2                       33.7                           -- 64.9                                  -- -- --  0.3    __________________________________________________________________________

                                      TABLE 2    __________________________________________________________________________    Proportional ratio (% by weight)     Sum of talc +                  Aluminum               kaolin +    Talc      Kaolin                  hydroxide                       Calcined                            Calcined     aluminum hydroxide    (BET)     (BET)                  (BET)                       talc kaolin                                 Alumina                                      Silica                                         (% by weight)    __________________________________________________________________________    Example 1          38.0              20.0                  22.0 --   15.0 --   5.0                                         80.0          (12.5)              (16.2)                  (15.8)    Example 2          38.0              10.0                  22.0 --   25.0 --   5.0                                         70.0          (12.5)              (16.2)                  (15.8)    Example 3          39.0              10.0                  16.0 --   25.0 5.0  5.0                                         65.0          (12.5)              (16.2)                  (15.8)    Example 4          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0           (7.0)              (16.2)                  (15.8)    Example 5          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (10.0)              (16.2)                  (15.8)    Example 6          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (17.8)              (16.2)                  (15.8)    Example 7          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (12.5)              (14.0)                  (15.8)    Example 8          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (12.5)              (16.2)                  (15.8)    Example 9          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (12.5)              (19.6)                  (15.8)    Example 10          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (12.5)              (22.0)                  (15.8)    Example 11          38.0              10.0                  22.0 --   25.0 --   5.0                                         70.0          (12.5)              (19.6)                   (6.2)    Example 12          38.0              10.0                  22.0 --   25.0 --   5.0                                         70.0          (12.5)              (19.6)                  (10.3)    Example 13          38.0              10.0                  22.0 --   25.0 --   5.0                                         70.0          (12.5)              (19.6)                  (18.0)    Example 14          39.0              22.0                  18.0 --   --   11.0 10.0                                         79.0          (12.5)              (19.6)                  (15.8)    Example 15          39.0              22.0                  18.0 --   --   11.0 10.0                                         79.0          (12.5)              (16.2)                  (10.3)    Example 16          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (12.5)              (19.6)                  (15.8)    Example 17          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0          (12.5)              (19.6)                  (15.8)    __________________________________________________________________________

                                      TABLE 3    __________________________________________________________________________    Proportional ratio (% by weight)     Sum of talc +                  Aluminum               kaolin +    Talc      Kaolin                  hydroxide                       Calcined                            Calcined     aluminum hydroxide    (BET)     (BET)                  (BET)                       talc kaolin                                 Alumina                                      Silica                                         (% by weight)    __________________________________________________________________________    Comparative          39.0              10.0                  11.0 --   25.0 10.0 5.0                                         60.0    Example 1          (12.5)              (16.2)                  (15.8)    Comparative          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0    Example 2           (6.8)              (16.2)                  (15.8)    Comparative          39.0              19.0                  12.0 --   25.0 5.0  -- 70.0    Example 3          (12.5)              (12.8)                  (15.8)    Comparative          38.0              10.0                  22.0 --   25.0 --   5.0                                         70.0    Example 4          (12.5)              (19.6)                   (4.8)    Comparative          19.5              21.0                  17.0 19.5 19.0 4.0  -- 57.5    Example 5          (12.5)              (19.6)                  (15.8)    Comparative          39.0              --  28.0 --   20.0 3.0  10.0                                         67.0    Example 6          (12.5)  (15.8)    Comparative          39.0              27.5                  --   --   18.5 15.0 -- 66.5    Example 7          (12.5)              (19.6)    __________________________________________________________________________

With each clay mixture, a cylindrical honeycomb structure (honeycombcompact) with a 103 mm diameter and a 120 mm height, having a squarecell shape with a 76 μm rib thickness and a 62 pieces/m² cell number wasformed by a known extrusion method according to the firing conditionsshown in Table 4 and 5 (Examples 1 to 17, Comparative Examples 1 to 7).

Measurement results of the coefficient of thermal expansion (thedirection of flow passages of the honeycomb structure) at 40 to 800° C.,the porosity, the thermal shock resistance and the isostatic strength ofthe obtained honeycomb sintered body (honeycomb structure) are shown inTables 4 to 5. FIG. 7 is a photograph showing the texture of the ceramicmaterial, which is the end face structure of the honeycomb structure ofExample 12. FIG. 8 is a photograph showing the texture of the ceramicmaterial, which is the end face structure of the honeycomb structure ofComparative Example 1.

                                      TABLE 4    __________________________________________________________________________    Firing conditions    Temperature             Honeycomb sintered body characteristics    rise rate               Coefficient    (° C/H)          of thermal                                      Thermal    average value                 Maximum                       Retention                            expansion impact                                           Isostatic    between value                 temperature                       time (10.sup.-6 /° C.)                                  Porosity                                      resistance                                           strength    to 1350° C.                 (° C.)                       (H)   40-800° C.!                                  (%) (° C.)                                           (kg/cm.sup.2)    __________________________________________________________________________    Example 1          60     1420  4    0.5   31  850  52    Example 2          60     1420  4    0.5   33  850  43    Example 3          60     1420  4    0.6   30  800  11    Example 4          50     1420  4    0.5   31  850  12    Example 5          50     1420  4    0.6   30  800  20    Example 6          50     1420  4    0.7   30  750  48    Example 7          60     1415  6    0.7   32  750  13    Example 8          60     1415  6    0.6   30  800  57    Example 9          60     1415  6    0.5   26  850  65    Example 10          60     1415  6    0.4   24  900  35    Example 11          60     1420  5    0.5   32  850  19    Example 12          60     1420  5    0.5   30  850  46    Example 13          60     1420  5    0.3   28  950  66    Example 14          60     1425  4    0.5   36  850  71    Example 15          80     1400  6    0.6   38  700  12    Example 16          60     1390  4    0.9   32  650  37    Example 17          80     1420  4    0.5   30  850  65    __________________________________________________________________________

                                      TABLE 5    __________________________________________________________________________    Firing conditions    Temperature             Honeycomb sintered body characteristics    rise rate               Coefficient    (° C/H)          of thermal                                      Thermal    average value                 Maximum                       Retention                            expansion impact                                           Isostatic    between 1000 temperature                       time (10.sup.-6 /° C.)                                  Porosity                                      resistance                                           strength    to 1350° C.                 (° C.)                       (H)   40-800° C.!                                  (%) (° C.)                                           (kg/cm.sup.2)    __________________________________________________________________________    Comparative          60     1420  4    0.7   29  750  8    Example 1    Comparative          50     1420  4    0.5   31  800  8    Example 2    Comparative          60     1415  6    0.8   33  700  7    Example 3    Comparative          60     1420  5    0.6   34  800  8    Example 4    Comparative          60     1425  4    0.9   40  650  *Not evaluated    Example 5    Comparative          60     1425  4    0.7   35  750  *Not evaluated    Example 6    Comparative          60     1425  4    0.6   33  800  5    Example 7    __________________________________________________________________________     *Not evaluated: a continuous honeycomb structure external wall cannot be     obtained (it is assumed to be 5 kg/cm.sup.2 or less).

(Evaluation)

As apparent from the results of Examples 1 to 17 and ComparativeExamples 1 to 7, the coefficient of thermal expansion and the porositycan be in a predetermined range, and can have excellent thermal shockresistance and isostatic strength when 65% or more cordierite rawmaterial including talc, kaolin and aluminum hydroxide is contained inthe material batch and the BET specific surface area of the talc,kaolin, and the aluminum hydroxide is in the specific range of thepresent invention compared with the case outside the range.

(Examples 18 to 21, Comparative Examples 8 to 9)

With clay mixture used in Example 12 and Comparative Example 1, acylindrical honeycomb structure (honeycomb compact) with a 103 mmdiameter and a 120 mm height, having the cell structure shown in Table 6was formed by a known extrusion method according to the firingconditions shown in Table 6 (Examples 18 to 21, Comparative Examples 8to 9).

Measurement results of the isostatic strength of the obtained honeycombsintered body (honeycomb structure) are shown in Table 6.

From the results shown in Table 6, it was learned that the isostaticstrength can differ drastically between the case with 65% by weight ormore cordierite raw material including talc, kaolin and aluminumhydroxide in the material batch as in the present invention and the casewith less than 65% by weight, even if the honeycomb structures with thesame wall thickness and cell density are produced. That is, when theweight ratio is outside the range of the present invention (less than65% by weight), the isostatic strength is less than 10 kg/cm², which isnot-sufficient for the practical use.

                                      TABLE 6    __________________________________________________________________________    Wall       Cell Isostatic                         Cordierite    thickness  density                    strength                         material    (μm)    (cell/cm.sup.2)                    (kg/cm.sup.2)                         batch Firing conditions    __________________________________________________________________________    Example 18          102   62  79   Example 12                               Temperature rise rate: 60° C./H*    Example 19          76    93  85         Maximum temperature: 1420° C.    Example 20          44   140  71         Retention time: 5 H    Example 21          40   186  88    Comparative          102   62   9   Comparative                               Temperature rise rate: 60° C./H*    Example 8            Example 1                               Maximum temperature: 1420° C.    Comparative          44   140   7         Retention time: 4 H    Example 9    __________________________________________________________________________

As heretofore mentioned, according to a production method of acordierite honeycomb structure of the present invention, a thinner walland a higher cell density can be realized in the cordierite honeycombstructure as well as the formability (in particular, the lubricity andthe shape stability) in extruding, the mechanical strength (hydrostaticpressure fracture strength) and the catalyst carrying characteristicscan be improved.

What is claimed is:
 1. A production method of a thin wall cordierite honeycomb structure having cordierite as the main component of the crystalline phase, which comprises:adding a forming auxiliary agent to a cordierite material to obtain a mixture, kneading the mixture to obtain a material batch, forming the material batch by extrusion to form a honycomb compact, drying the honeycomb compact to obtain a dried body, and firing the dried body, wherein 65% by weight or more flat plate-like cordierite raw material including crystal water made from talc, kaolin and aluminum hydroxide is contained in the cordierite material batch, with the BET specific surface areas for the cordierite raw materials being 7 to 18 m² /g of talc, 14 to 22 m² /g of kaolin and 6 to 18 m² /g of aluminum hydroxide.
 2. The production method of a thin wall cordierite honeycomb structure according to claim 1, wherein the average wall thickness of the honeycomb structure is 110 μm or less.
 3. The production method of a thin wall cordierite honeycomb structure according to claim 1, wherein the coefficient of thermal expansion in the direction of flow passages of the cordierite honeycomb structure at 40 to 800° C. is 0.8×10⁻⁶ /°C. or less, the porosity is 24 to 38%, and the isostatic strength is 10 kg/cm² or more.
 4. The production method of a thin wall cordierite honeycomb structure according to claim 1, wherein the BET specific surface areas for the cordierite raw material are 10 to 18 m² /g of talc, 16 to 20 m² /g of kaolin and 10 to 18 m² /g of aluminum hydroxide.
 5. The production method of a thin wall cordierite honeycomb structure according to claim 1, wherein 70% by weight or more flat plate-like cordierite raw material including crystal water made from talc, kaolin and aluminum hydroxide is contained in the cordierite material batch.
 6. The production method of a thin wall cordierite honeycomb structure according to claim 3, wherein the porosity of the honeycomb structure is 30 to 38%. 